The energy consumption of industrial boilers accounts for a significant portion of the total energy use within the industrial sector. One area of significant energy loss can occur where pipes enter and leave (i.e., penetrate) the boiler casing. These areas can create hot spots and thermal loss. Moreover, these areas include joints that represent vulnerabilities that can result in unplanned shutdowns and/or risks to operational and personnel safety. Thus, the penetrations in these areas must be protected with seals to ensure the efficient, safe operation of industrial boilers.
In some instances, the industrial boilers must use seals that are designed to be installed during the construction phase of a project (i.e., before the pipes are closed and tested). Good engineering practice dictates that these seals have a theoretical life that is equal to or greater than the plant in which the seals are to be installed.
Existing standards dictate that, once an industrial boiler system is in operational mode, any mechanical rework, such as grinding or welding on pipes carrying superheated steam, must be heat treated and/or stress relieved before the system can be put back in service. As a result, the replacement of a failed seal for such a system can be a prohibitively expensive procedure.
One technique that can be used to protect a boiler casing joint is to use a fabric conical seal that attaches to a pipe and to a standoff with clamps. A standoff is a raised structure that is welded to a casing plate that surrounds a pipe that penetrates through the casing plate. The fabric conical seal produces a seal that is inexpensive, has zero deflection forces, and is easy to install. However, such seals are susceptible to damage, have poor sealing characteristics, and are suitable for low temperature applications only.
A bellows-type seal can also be used to protect boiler casing joints. Such seals are made of thin gage stainless steel that is formed into convolutions. One end of the convolution is welded to an internal ring. The other end of the convolution is welded to an external ring. The internal ring can be welded to the outside of the pipe. The external ring can be welded to a casing plate that is penetrated by the pipe.
The repair technique for the bellows uses a clamshell design. The clamshell design involves manufacturing a replacement bellows-type seal and cutting it into two pieces along a length. The two pieces are placed around a pipe and are welded together before end rings can be attached. The end rings are an internal ring for the pipe and an external ring to the casing plate. Also, the spring rate (i.e., the force to deflect the bellows) of a clamshell creates an additional external force, which may be undesirable.
The advantages for these bellows-type seals are that they are inexpensive and simple to install. They also have good sealing characteristics and do not require service. However, bellows-type seals are susceptible to mechanical damage and cannot be replaced easily.
A packed-type seal is another type of seal that can be used to protect boiler casing joints. Packed-type seals include discs of insulation that are packed around a pipe at the point at which the pipe penetrates through the casing wall. The discs can be held into place with a bolted seal plate or flange. The insulation can be made from mineral wool, ceramic fiber, or other similar materials.
The packed-type seals can be inexpensive, easy to install, and have zero deflection forces. However, the seals can be susceptible to damage and can have poor sealing characteristics.
Another technique for sealing a boiler casing joint includes using a multi-laminar fabric seal. Such seals include a standoff that is welded to a casing plate, a pan weldment that is welded to the outside of a pipe, mineral wool or other similar insulation material for the void, and a belt-type seal which is attached with clamp bars.
The advantage of multi-laminar fabric seals is that the seals can have minimal deflection forces and can be installed after the system is put in service. Also, such seals can be used for multiple penetrations. However, such seals are expensive, bulky, difficult to install, and are susceptible to mechanical damage. In view of the above, there is a need for an improved penetration seal for boiler casings.